The present invention relates to the froth flotation of finely-divided coal particles for separation of the ash therefrom and more particularly to a new frothing agent or frother which enhances coal recovery in the froth flotation process.
In the coal industry, various types of shale and clay are produced as a mixture with the coal. To increase the heating value of the coal and to reduce the hauling costs, a complex process of coal washing is normally used to reduce the total ash content. In this process the coal is graded to a certain size, usually less than six inches, and then fed into a slurry bath in which the density of the media is closely controlled. The coal floats in the heavy media bath while the heavier rocks sink to the bottom. Following this heavy media separation, all the floated material is again sized by vibrating screens for further purification. The smaller size fraction may be processed by shaking tables, hydrocyclones, or froth flotation. In each of these steps, coal is recovered and dried prior to shipment.
Flotation is a process for separating finely ground minerals such as coal particles from their associate waste or gangue by means of the affinity of surfaces of these particles for air bubbles, which is a method for concentrating coal particles. In the flotation process a hydrophobic coating is placed on the particles which acts as a bridge so that the particles may attach to the air bubble and be floated, since the air bubble will not normally adhere to a clean mineral surface such as coal.
In the froth flotation of coal, a froth is formed by introducing air into so-called pulp which contains impure finely divided coal particles and water containig a frothing agent. The flotation separation of coal from the residue or gangue depends upon the relative wettability of surfaces and the contact angle, which is the angle created by the solid air bubble interface.
A frothing agent is utilized to provide a stable flotation froth persistent enough to facilitate the coal separation but not so persistent that it cannot be broken to allow subsequent handling.
Froth flotation is performed in machines specifically designed for the purpose, i.e., the Denver Sub-A machine, and the Wemco machine.
The use of froth flotation to effect a separation of ash particles from coal can be achieved only if liberation of these unwanted particles from the coal has taken place. Most high-grade coals are floatable naturally due to their hydrophobic surface and typically only require a frothing agent for effecting flotation. A frothing agent imparts elasticity to the air bubble, enhances particle-attachment so that the coal is buoyed to the surface of the slurry.
Conventional frothing agents or frothers in the coal flotation process generally have been alcohols and ethers, such as 2-ethyl hexanol, short-chain alkanols, terpene alcohols such as alpha-terpineol, short-chain glycols, sorbitol derivatives, ethoxylated alcohols, mixed alkylene oxide glycol ethers, and alcohol frothers comprising the reaction product of a C.sub.5 -C.sub.10 diol and a compound selected from the group consisting of an alkylene oxide and an acrylonitrile. See U.S. Pat. No. 4,606,818 (Keys), issued August 19, 1986.
One problem associated with the conventional frothing agents set forth above is that alcohols, such as 2-ethyl hexanol, are believed to be teratogenic. Use of such alcohols as frothing agents may be hazardous to those who come in contact with such agents and may eventually be banned from use. Therefore, the present inventors have developed a novel froth flotation process which utilizes a unique group of frothing agents or frothers.
These novel frothers overcome the many disadvantages of the aforementioned conventional frothers, as well as provide the following advantages: increased clean coal recovery, a frother having both frother and collector properties when used in coal flotation processes, and avoids the potential health hazards believed to be associated with conventional 2-ethyl hexanol containing products.
Additional advantages of the present invention shall become apparent as described below.